This invention relates to infrared furnaces and, more particularly, to an infrared furnace suitable for firing electronic components in a nonreactive environment.
Application Ser. No. 306,200, filed Sept. 28, 1981, the disclosure of which is incorporated fully herein by reference, describes a method for firing thick film electronic circuits and an infrared furnace in which such method can be carried out.
In this furnace, an insulated firing chamber has oppositely disposed sidewalls with a plurality of aligned pairs of holes. Infrared lamps are installed in the chamber. The end terminals of the lamps pass through the respective hole pairs in the sidewalls of the chamber to the exterior of the chamber, so the end terminals are not exposed to the high temperature in the firing chamber. Because of the high temperatures, it is difficult to seal effectively the sidewalls where the lamps pass through the hole pairs, with the result that atmospheric air leaks into the firing chamber.
In application Ser. No. 381,901, filed May 25, 1982, the disclosure of which is incorporated fully herein by reference, the described infrared furnace is provided with a firing chamber having an elongated, tubular muffle transparent to the infrared energy. The components being fired pass through the muffle and are thus directly exposed to the short wavelength energy emitted by the infrared lamps, which lie outside the muffle. The ends of the muffle lie outside the firing chamber in sealed chambers to prevent atmospheric air from entering the muffle, while a nonreactive gas passes through the muffle to sweep away volatiles released during the firing operation. In this manner, a controlled environment can be established for the components being fired. Typically, the oxygen content in the envelope can be kept to 10 ppm or less. The presence of the envelope in the firing chamber, however, produces a reduced cross-sectional area through which the nonreactive gas can flow. This has a tendency to create turbulence in the muffle. However, turbulence is an undesirable condition because it disturbs the planned temperature profile. The reduced cross-sectional area also increases the tendency for volatiles to condense in the muffle. Finally, the walls of the muffle absorb some of the infrared energy, thereby reducing somewhat the efficienty of the heat transfer to the product being fired.